Typically, cardiovascular pressures are measured using catheters which are introduced into the vascular systems via an artery or vein. Catheters exhibit a finite risk of both morbidity and mortality with routine usage in the clinical situation. More recently, sensor tipped guidewires to measure pressure have been developed. However, this procedure is also invasive with concomitant associations of morbidity and mortality. There are currently no adequately accurate direct, noninvasive real time clinical methods used to measure pressure in the cardiovascular system. An indirect method exists using Doppler ultrasound as suggested by Laaban, et al. (Laaban, J., Diebold, B., Zelinski, R., Lafay, M., Raffoul, H., and Rochemaure, J., Chest 96, (6): 1258-1262, 1989). With Doppler techniques, blood flow velocities are measured using ultrasonic scanners operating in the Doppler mode. By applying the Bernoulli equation and knowing the peak velocity, it is possible to calculate the pressure drop across a cardiac valve that created the flow. If one starts measurements in a vein such as the superior vena cava of known low pressure, one can calculate the pressure in the right ventricle and pulmonary artery and even make an estimate of the endiastolic left ventricular pressure with the technique. The indirect approach is filled with errors in difficult cases when good data is most needed and is used only for diagnosing the right side of the heart. Other non-invasive measurement schemes have been proposed (Blazek; Vladimir, Schmitt; Hans-J., U.S. Pat. No. 5,447,161; Aakhus, S., Soerlie, C., Faanes, A., Hauger, S. O., Bjoemstad, K., Hatle, L., and Angelsen, B. A. J., American Journal of Cardiology, 72: 260-267, 1993; Kyriakides, Z. S., Kremastinos, D. T., Rentoukas, E., Vavelidis, J., Damianou, C., and Toutouzas, P., International Journal of Cardiology, 33: 267-274, 1991; Neuman, A., Soble, J. S., Anagnos, P. C., Kagzi, M., and Parrillo, J. E., Journal of the American Society of Echocardiography, 11(2): 126-131, 1998) but show no significant advancement to the field.
In U.S. Pat. No. 3,640,271 to Horton, there is presented the concept of injecting a single bubble of known size into a patient for the purpose of measuring blood pressure. The concept was to stimulate the bubble into resonance ultrasonically and from the received backscattered signal, determine the resonant frequency of the bubble. It is further known that if both the diameter of the bubble and the resonant frequency are known, then the unknown pressure could be calculated. However, it is not known that the precision sized bubbles required for the technical approach have ever been achieved. Also, at present, it would be nearly impossible to locate bubbles within a specific organ or cavity for the very low concentration of bubbles required by the technology.
In Pat. No. 4,265,251 to Tickner, the concept of encapsulating a pressurized bubble within a fused saccharide shell is presented. The shell begins to dissolve in the circulatory system, thinning the wall. At some point in its dissolution, the shell fractures and the bubble escapes and expands. In so doing, it over-expands from its encapsulated diameter which sets it to free-ringing. A passive external transducer detects the free ringing signals and, by applying the same equations identified by Horton, computes the pressure. A limitation to the technology for usable clinical practice is the inability to control the point of rupture and the lack of precision (Osterle S., Sahines, T., Tucker, C., Tickner, E., et al., The Western Journal of Medicine, 1985 Ott; 143: 463-468.
In U.S. Pat. No. 5,749,364 to Sliwa, the concept for mapping cardiac pressures is presented by injecting a population of non-precision microspheres into the blood pool. Theory indicates that the resonant frequency peak of an encapsulated bubble is mathematically related to the ambient pressure. By examining the backscattered signal of the microspheres and from the change in their frequency spectrum, a map of the pressure in at least two dimensions is derived. One claimed method for doing this is to inject two microsphere population types, which exhibit different backscatter characteristics, and then use these different characteristics to deduce ambient pressures. Other work in using frequency shift has been explored. However, no clinical applications are known to have been developed (Ishihara, K., Kitabatake, A., Tanouchi, J., Fujii, K., Uematsu, M., Yoshida, Y., Kamada, T., Tamura, T., Chihara, K., and Shirae, K., Jpn. J. Appl. Phys., 27(Suppl 27-1): 125-127, 1988) possibly due to the difficulties in measuring in-vivo frequency shifts. Furthermore, commercial ultrasound scanners have relatively narrow frequency bandwidths, which would not allow for the frequency range scan needed to detect changes in resonant frequency, especially in a formulation with multiple microsphere populations with attendant multiple resonance peaks.
In PCT No. 98/32378 (De Jong, N., Frinking, P., PCT No. WO 98/32378, Jul. 30, 1998; Bouakaz, A., Frinking, P., De Jong, N., Non-Invasive Pressure Measurement in a Fluid Filled Cavity, Abstract: The Fourth Heart Centre Symposium on Ultrasound Contrast Imaging, Jan. 21-22, 1999) there is disclosed the use of the decay of free gas bubbles to measure ambient pressure or temperature. The decay time of the gas bubble is dependent on the gas type, the liquid characteristics, the solubility of the gas within the liquid, the excitation frequency and the ambient temperature and pressure. However, in their scheme the microsphere is used only as a transport mechanism which releases the free bubble upon insonation and the properties of the free bubble are utilized for pressure measurement. They propose using a series of intermittent high power pulses to break the capsule of the microsphere, releasing the gas bubble and then using a series of intermittent low power pulses to determine the decay time of the bubble and therefore calculate pressure or temperature. Although the mechanism described uses a power level to rupture the capsule that is above a threshold, the fragility or release mechanism of the microsphere capsule itself is not controlled. Furthermore, if the microsphere has a very weak capsule, breakage of the capsule can occur at very low ultrasound powers. This leads to a response which is not controlled relative to imaging depth and applied power.